1. Field of the Invention
The present invention is related to an image signal compressing device, especially to an improvement of the processing speed at which image information, which has been compressed according to a JPEG (Joint Photographic Expert Group) algorithm and has been recorded in a recording medium, is restored to the original image information using two dimensional IDCT (inverse discrete cosine transformation).
2. Description of the Related Art
A standard algorithm for the encoding of high resolution images and the transfer of that information through telecommunication transmission channels has been recommended by the JPEG. In the algorithm recommended by the JPEG, i.e., the base line process of the JPEG algorithm, to enable a large-scale data compression, first the original image data are broken down into components on the spatial frequency axis using two-dimensional DCT (discrete cosine transformation). Thereafter, data expressed on the spatial frequency axis is quantized using a quantization table. The quantized data is then encoded using a Huffman table.
When the original image is to be restored from the image information which has been encoded and recorded in the recording medium, the encoded data is decoded and inversely-quantized, and then, the two-dimensional IDCT is applied to the inversely-quantized data.
In the inverse-quantization and in the IDCT, an enormous number of multiplications and addition-subtraction calculations are carried out. The multiplication process is more complex than the addition-subtraction calculations, and therefore, the number of multiplications has a greater effect on the calculation time.